SSL active but still not secure connection through FM on Mac

we are using our own URL, managed on united-domains.de, which points to the AWS URL. There is a SSL certificate attached to the URL on United Domains and it works perfectly for all connections in the browser. However, if I connect with the Filemaker App on my Mac I get this message. What am I doing wrong?

I had a similar issue with a client, and in the end, it turned out that the issue was that the intermediate certificate hadn't been installed properly by the person who did the initial installation. I'm not 100% sure that this is the explanation in your case, but what worked for me was to re-install the certificate by exactly following the steps documented here: FileMakerPKB

What @David says is true. To add if you dont have the intermediate certificate you can grab it from the Signed Certificate File. and then insert it in de admin console. And do you use the same url as the certificate states?

we are using our own URL, managed on united-domains.de, which points to the AWS URL

Can you expand a bit on this?

If this something like URL forwarding then it may not work because the SSL cert may be for something.somewhere.com but the actual address that the final connection happens on may be 12345.amazonaws.com, which will not match the SSL cert?

We can't tell from the screenshot, but is the part that is blotted out, the actual name on the cert?